The invention relates to a tubular bicycle fork pivot comprising a cylindrical wall which has a longitudinal geometric axis.
It is known that in bicycles, particularly competition bicycles, weight is an important factor, particularly when traveling over uneven or mountainous terrain. Attempts have therefore been made at lightening the overall weight by reducing the weight of the various components that make up a bicycle. It is, however, necessary that such a reduction in weight should not be accompanied by an unacceptable reduction in the mechanical strength of the bicycle component concerned.
The applicant company has thus developed a bicycle fork made of composite material, according to Patent FR-B-2 684 062, filed on Nov. 25, 1991, which is entirely satisfactory and which enjoys widespread use in very high-level cycling competitions.
It is recalled that the term xe2x80x9ccomposite materialxe2x80x9d is used to describe a material which is made up of fibers of high mechanical strength, particularly carbon fibers and/or glass fibers, generally in the form of laps, which during a molding operation are embedded in a resin of the epoxy or polyester or equivalent type, which then hardens.
This fork of the prior art is combined with a metal fork pivot in order to exhibit the desired strength properties. However, this metal fork pivot, generally made of alloyed steel, adds weight to the assembly.
Tests have been carried out in an attempt to make the fork pivot out of composite material in order to save weight. These tests have shown that the overall performance of the fork equipped with such a pivot constitutes a retrograde step, both in terms of the frontal strength (that is to say the ability to withstand the efforts involved in braking and longitudinal running jolts which are due to the unevenness of the surface being ridden over) and in terms of the lateral strength (that is to say the ability to withstand transverse efforts, particularly when the cyclist is riding out of the saddle, tilting the bike from side to side.
The size of the pivot, particularly its outside diameter, is governed by the accessories that will be assembled on top of it. One such accessory is the headset bearing cup, which has a standardized diameter of 26.5 mm. Another such accessory is handlebar stems of the xe2x80x9cthreadedless headsetxe2x80x9d type, with a bore diameter of 25.4 mm which, unlike handlebar stems which are fixed into the pivot tube by an expander which locks inside the pivot tube, surround the outer surface of the cylindrical wall of the pivot and are locked in place by clamping around this wall.
As the interior volume of the tubular pivot is unoccupied, because the stem is no longer housed therein, attempts have been made at increasing the thickness of the cylindrical wall of the pivot made of composite material, toward the inside, with a view to obtaining performance that is equivalent to that of a metal pivot.
The results obtained with such a pivot made of composite material but with a thicker wall are not satisfactory.
The object of the invention is, above all, to provide a bicycle fork pivot which, while being more lightweight than the conventional metal pivots, allows mechanical strength performance at least equivalent to that of metal pivots to be obtained.
It is furthermore desirable for such a pivot to remain relatively simple and as economic as possible to manufacture.
According to the invention, a bicycle fork tubular pivot comprising a cylindrical wall which has a longitudinal geometric axis is characterized in that it is made of a composite material and comprises at least one longitudinal internal reinforcing partition.
Advantageously, the pivot comprises a partition parallel to the plane of travel of the bicycle (the mid-plane of the forks), this partition being capable of absorbing frontal stresses, and a partition orthogonal to the previous one and capable of absorbing lateral stresses.
As a preference, each longitudinal internal partition passes approximately through the longitudinal geometric axis of the pivot.
The pivot preferably comprises at least one longitudinal partition extending along a diameter.
The pivot may comprise several longitudinal radial partitions distributed angularly about the geometric axis and extending approximately between this axis and the cylindrical wall, these partitions being linked together in the region of the longitudinal geometric axis.
Two successive radial partitions advantageously constitute the approximately flat faces of an elemental prism, the cross section of which is in the shape of a circular sector, this prism having a convex outer face.
Each prism consists of laps of fibers wound around the contour of the corresponding sector.
The number of radial partitions may be an even number, the radial partitions being approximately diametrically opposed and aligned in pairs, a group of two radial walls thus aligned forming a diametral partition.
An outer cylindrical winding of laps of fibers surrounds the convex faces of the elemental prisms.
The pivot may be secured to an insert made of composite material, in the shape of an inverted U, for connecting to the fork crown. This insert may also comprise at least one internal reinforcing partition.
The pivot is advantageously manufactured around a mandrel or core split into a cross shape in the region in which the partitions are produced. The dimensions of the partitions may change according to the size of frame to be equipped.
The structure of the pivot and of the partitions consists of a textile armature made up of two-way and one-way fibers of the same type as those that make up the remainder of the fork.
A preform is made and placed in a tool which has the final dimensions of the part. The fibers may be preimpregnated with resin. The molding operation is performed next. If the fibers are not preimpregnated, a resin (epoxy, polyester or equivalent) is injected through the mold. The resin impregnates all of the structure thus binding all the constituent parts together. This is then cured by raising the temperature of the assembly (depending on the type of resin used).
After cooling, the fork is released from the mold.
The fork may be manufactured in a single molding operation involving the pivot, the insert, the fork crown, the fork blades and the dropouts.